Well Digimarc has been a preinstalled Photoshop plugin for ages. Though the version in PS(E) only reads the watermark, though for watermarking you need to give out some cash. Evidently ( tested by RC Concepcion) it is possible to read the invisible watermark even after printing and rescanning.

We have that on official government documents in the UK, (old driving licence counterpart etc), but this worked by cleverly printing the words on the paper in a way that it was very hard to detect with the naked eye, but would be picked up easily by a photocopier or scanner. I've never seen it used in the way you want, and am supprised that you say it can be. I would think the government spent a small fortune in having it incorporated in documents, on a piece by piece basis.

Does anyone know what kind of software to use to make invisible watermarks on photos? Alot of what I've found is for visible watermarks.

Specifically what I am after, is something to make an invisible watermark "copyright", that can't be seen TILL copied/scanned. We went on a cruise and got pictures taken one time that were like this, you could not copy them, as an invisible watermark would show up "ruining" the picture /copy if you did. I am looking for something that would do this, so that I can give pictures to someone without risk of them copying them.